The present invention relates to robotics, and more specifically to a practical, self-contained robotic device that in one form closely simulates a human hand in dexterity, as well as an anodizing process used in making the robotic device.
Robots hold the promise of relieving workers from dirty and dangerous tasks; improving the quality of products; improving the efficacy of surgical procedures; and liberating the aged and infirmed with the dignity of self-sufficiency. And, in a world economy increasingly dependent on productivity, robotics holds the promise of a critically relevant technology.
Robotics, however, at present does not contribute significantly to the economy and society. For example, within the present US approximately $10 trillion economy, the robotics market is only about $1.2 billion (annual installations measured at the peak of the recent economic boom). That is only 1/100th of one percent. By comparison, the scented-candle market in the U.S. is presently about $3 billion.
As a symptom of their failure, robots tend to execute only repetitive tasks, such as: go to point A, close gripper, go to point B, release gripper, repeat, repeat, repeat. Industrial robots spend their entire lives repeating steps like these, making them expensive alternatives for the equally effective dedicated machinery that they replace.
Herein, the broadest meaning of “robotic arm” is used to include any mechanical means of transporting a robotic tool or robotic hand to perform one or a number of tasks. The term applies whether the mechanical means is made of any combination of serial and parallel link(s), whether it includes any combination of rotating and sliding joint(s), or whether combining motion along tracks. The term also includes transport by robotic vehicles, whether tracked, wheeled, legged, water-borne, air-borne, space-borne whether or not combined with the above forms. From a controls standpoint, the transporting means can be autonomous (guided by machine intelligence), teleoperated (guided by human intelligence), or any combination of autonomous and teleoperated control.
Herein, the broadest meaning of “robotic hand” is used to mean any practical tool that performs mechanical functions, such as gripping, grasping and/or manipulating objects in its environment.
The purpose of robotic systems is to do useful tasks. In a robotic system, it is the tool and not the robotic arm that interacts directly with the task. The tool faces complexities and variations that are unique to every task object. Distinguishing between the robotic arm and the tool it carries is not arbitrary. Rather, market forces drive this split. The dividing plane between the robotic arm and the tool is sharply demarked by the outside face of the last component on the robot arm—the tool plate—a circular plate capping the last link with screw threads and alignment features for securely fastening the tool.
On the robot side of this face is the robotic arm and its supporting electronics and controllers. These robotic arms are mass-produced by one of a dozen or so multinational robotic-arm manufacturers that compete on slim margins with economies of scale. These companies have avoided “one-off” customization by nurturing a cottage industry of local systems integrators who customize robot trajectories in software and customize the tools in metal.
Robotic hands can be classified into three levels of sophistication.
The least sophisticated robotic hand has pincer fingers or “jaws”, typically two or three in number, that close and open to clamp and release objects of similar geometry based on external actuation, such as pneumatic pressure. This type of hand is used commonly in commercial and industrial robotics. Often the finger surfaces are custom shaped for a particular object in a fixed orientation. “Soft jaws” are often used to effect this customizing. They are pieces of a readily machinable metal (aluminum or steel) or other structural material, replaceably secured to pincers or articulated fingers to effect this tailoring. Then, to handle more than one shape, or to handle one shape in a variety of orientations, the operator commonly uses a tool changer to switch from one hand to another hand with different customized finger shapes.
Hands that can grasp are the next more sophisticated. They typically have articulated “fingers” that can wrap around an object, not merely clamp it.
The next most sophisticated robotic hand can not only grip and grasp, but has a dexterity, through multiple degrees of freedom, and multiple articulated links, that can also manipulate objects that are gripped or grasped. The most sophisticated robotic hand has motors and sensors synthesized by machine intelligence with electronic communications to other devices. The mechanical actions can arbitrarily grasp and/or manipulate a variety of objects of different shapes, sizes, and other varying combinations of physical properties.
The term “practical” provides a very important distinction between hands that can function in a laboratory setting, or in some highly specialized environment, ones that will be termed herein “academic”, and hands that are sufficiently compact, robust (rugged and durable), modular, lightweight, and cost effective to be useful commercially and in industrial applications. For example a practical hand should be light enough to be attached to commercially available robotic arms without reducing the resulting effective payload rating to zero. In conventional industrial robotics, a controller box is located on the floor near, or built into, the base of the robot arm. The robotic arm ends in a tool plate. Robotic arms typically weigh 50-100 times their rated payloads, so that a 10 kg payload requires a robotic arm weighing nearly a metric ton. A practical hand should also be modularly attachable to the robotic tool-plate.
“Practical” also means that the hand is suitably designed to survive the demands and environmental conditions of its intended task. The restriction on practicality excludes all but the simplest previous robotic hands that, while otherwise sophisticated or dexterous, were never intended for practical use on a commercial robotic arm. These several dozen non-practical robotic hands have been used in the academic study of the science of the force interactions of fingers against objects, mathematical analysis of grasp stability, and as engineering projects in graduate schools. Robot Evolution, The Development of Anthrorobotics, by Mark E. Rosheim (1994) gives an overview of such “academic” hands at pp. 195-225. Another dexterous hand, one developed at the University of Pennsylvania, is described in U.S. Pat. Nos. 4,957,320 and 5,501,498, both to Ulrich. To the best of applicants' knowledge, none of these hands is used commercially or industrially.
Compactness is another practical consideration, particularly as it relates to control. Articulated joints require the physical routings of wires for power and communication from a control box to the point(s) of articulation.
Typically, fat bundles of wire extend from the control box to the robot arm and an attached hand. For a practical hand, this means that bundles of wire are accommodated and routed through joints, and be subjected to millions of cycles of flexure. Design problems are increased, and durability (“robustness”) is seriously adversely affected. Also, a prototype dexterous hand is rendered impractical when its electronics and drive components are too bulky, requiring an increase in the height of the hand. The distance from the wrist center to the payload center, which is directly dependent on the hand height, degrades overall system performance in two ways. First, it reduces available torque at the robot joints, especially for the wrist, for any given payload. If the distance is doubled the allowable payload is halved. If the distance is zero, the effect on wrist torque is zero. Second, it limits the size of the dexterous workspace, so that simple wrist rotations increasingly require increasingly exaggerated motions of the biggest, heaviest arm links. The greater this motion, the more joint-drive-power required, the more effort required to avoid collisions, and the lower the margin of safety. In the ideal case of zero distance, pure rotations cause zero motion of the larger links.
Expanding on the limitations of the robot wiring, robot structures have to be long and slender in order not to interfere with their tasks or themselves. The slenderness is limited by the complex joints which must both support loads with bearing sets and impart torques with mechanical drives. As a rule, robot manufacturers do not ship robots with externally mounted wires. Electrical wires can be routed on the outside of a robot by users, but with huge cost, because the robot ends up wrapping and unwrapping the wires around the structure as the joints rotate. The motion can require several meters of active service loop; and even then, a small snag or even capstan effect can instantly sever them. The only safe option is to route the wires internally, but the wires need to allow flexure along each sequential joint axis. Furthermore, to reduce fatigue, expensive cable (“robot cable”) is installed with generous (volume-expensive coils) at each joint axis. The need to reserve free space at each joint axis for coils of wire, impacts the cost of the joint mechanisms severely, arguably creating the greatest single cost and performance impact on any robot, whether arm or hand.
“Practical” also involves interrelated considerations regarding industry-standard tool-plate location versus hand bulk versus wires. All major commercial robotic arms end in a tool plate that is located just after the wrist axes. The wrist and forearm of commercial arms are integral in terms of mechanical, electrical, software, control, and safety, and cannot be removed. They also cannot accommodate more than a couple air hoses or wires. The academic dexterous robotic hands include a large volume of motors and transmissions, typically directly behind the hand. Most researchers use these hands as tools to study machine manipulation without ever intending to mount their hand on the end of a robotic arm. While some researchers claim that their hands can be commercially viable, the usual suggestions are not in fact practical. One suggestion is of removing the arm's forearm and wrist and replacing them with an integrated forearm+wrist+hand assembly of the researcher's design. However, the industry firmly rejects removing the forearm and wrist. Another suggestion is to mount the volume of motors and amplifiers at the base of the arm and to run wires all the way through the robot's joints. But each brushless motor in hand requires at least 3 heavy-gage power phase leads, a heavy-gage safety ground, and 4-7 position-feedback leads for commutation. Any other sensors (force, temperature, vision, tactile, etc.) require additional wires to be threaded down the entire robotic arm. Typically these hands require 50-150 support wires. Additional dexterity requires additional motors and therefore proportionally more wires. Clearly, there is a need for hand dexterity that is independent from the number of wires.
Because of these requirements of a practical hand, the single most common tool is a gripper with 2 or 3 jaws, which is sold with the aforementioned “softjaws” made of aluminum or machinable steel. With varying degrees of success, the integrator applies experience and intuition in a time-consuming, iterative process to design the jaw shapes that will secure target objects reliably. For every unique variation in object size, shape, or orientation, a new tool is prepared and a tool exchanger employed to switch between this and other tools. Since the robotic-arm manufacturers and the tool integrators presently are independent business entities, the tool is designed as a self-contained module ready to be fastened to a tool plate or tool-exchange adaptor. Since the tool is located at the far end of the robot from its base, any tether for pneumatic, hydraulic, or electric control should be thin enough to fit (with other tethers) through restricted channels along the robot structure; flexible enough to face millions of flex cycles around multiple axes without fatigue failure; and robust.
The tether restriction limits the amount of sensor or control bandwidth that can be supported between the arm base and the tool.
The tool is attached at the end of a robotic mechanism capable of transporting the base of the tool with precision. (Whole-Arm Manipulation as described in U.S. Pat. No. 5,207,114 is the only case known to applicants in which other parts of the arm interact physically to achieve tasks.) Also, lasers, water-jet cutters, dispensers, and arc-welders do not make hard physical contact with the task, but they are nearest to the task and their trajectory controls the quality of the task. While the robotic transport mechanism is far bigger and more expensive than the tool, its only role is transportation of the tool. Otherwise the arm's own bulk obstructs valuable workspace, blocks access to the work piece, and introduces the dominant safety hazard. Tools, such as robotic hands, are part of a much larger system, such as a workcell (see FIG. 1), which exists mainly to impart intelligent motion at the base of the tool. Typical system components include:                an articulated robotic arm, with joints driven by electric motors        a set of motor-power amplifiers, normally mounted near the base of the robotic arm        a motion-control processor which coordinates the arm motor velocities        a processor to which the sensors report and which coordinates system activities        the object work piece on which the system operates to perform a task        various electronic sensors, measuring contact, vision, proximity, temperature, etc        
Typically one of the dozen or so multinational robot manufacturers provides the components that are readily mass-produced, including the arm(s), amplifiers, and motion-control processor(s). Then an integrator works with the end-user to specify sensors, customize the end-of-arm tool(s) for a specific task, and program the system. When multiple tasks are requested of a robotic system or there is significant variability in the task, then separate individual tools are customized and exchanged with a tool-changer for each part of the task. Individual tools are kept therefore on racks within reach of the arm. Since the tool customization process usually involves time-consuming machining, duplicate spares for each unique tool are kept in local inventory to minimize production down durations in case of a tool failure. In general, the more complex the task(s), the greater the reliance on both tool variety and sensor input.
It is easy to overlook the importance of the tool, given that it traditionally makes up roughly only 1% of system cost and 0.1% of system weight and has a level of sophistication amounting to one bit of control—full-open versus full-closed in the case of grippers.
A practical dexterous robotic hand would have many obvious and many subtle advantages over less-dexterous grippers. Mean-time-between-failures (MBTF) is a good example of a subtle, even counter-intuitive advantage. At first glance one might assume that MTBF of simple, low-part-count grippers would easily beat complex, dexterous hands. But grippers have no control or sensing, so their action runs full speed into mechanical stops on every cycle, concentrating failure there.
By contrast, if a hand is dexterous and intelligent through sensors and controllers, MTBF is not nearly as important as the standard deviation of MBTF. That is, it is far more important to know precisely when a particular unit needs servicing than to make the average time very long.
Another disadvantage for dexterous hands generally—as compared to grippers with pre-shaped gripper-jaw geometries—that goes counter to conventional wisdom is that dexterity does not provide position information to the same level as gripper jaws with limited motion and geometric locating features formed on the jaws of the grippers. A gripper (with shaped jaws) is programmed to pick up the same part at the same pick-up location over and over. As long as the part position error just before gripping is within the chamber size of the gripper jaw's geometric feature(s), then the part will adjust its location to fall precisely into that feature. The net result is that the act of gripping the part reduces its position error. This does not occur with known, academic dexterous hands.
In contrasting known dexterous hands to known grippers, it is also important to note the end-effector, aka the tool, of a robotic arm faces the greatest extremes of any robotic system. The extreme location of end effectors at the tip of the robotic arm has four consequences.                1. It is usually the first part of the robot to make impact with obstacles.        2. It is the fastest moving part of the robot.        3. It is nearest part of the robot to the extreme conditions that necessitated the use of a robot in the first place, like the pelting of molten weld splatter.        4. More than any other part, its mass (and that of any additional payload) requires a disproportionate fraction of joint torque.        
With or without machine-vision, robot arms crash end effectors into immovable obstacles. Most frequently, the immovable obstacle is the target payload or task itself. Errors in robot trajectories during programming and misplaced payloads after programming are the usual culprits. Generally, it is expected that the end effector will be designed to withstand these impacts or be easily (and cheaply) replaced. These considerations have also deterred the adoption of dexterous hands for commercial applications. In short, known dexterous hands are not practical.